The present invention relates to a method and apparatus for setting the preload between die plates such as found in molding, stamping and pressing machines. More particularly, the invention is concerned with setting the preload between two die plates that are operated by a clamping mechanism of given stroke.
In injection molding machines of the prior art, a pair of die plates are reciprocated relative to one another to open and close molds defined or carried by the plates. A toggle linkage is customarily employed to develop the high clamping forces holding the die plates together during the injection process. The linkage reacts the clamping forces through a thrust plate and a set of adjustable tie bars that extend between the thrust plate and the die plates. The clamping load is sometimes referred to as the preload because it is developed by the linkage as soon as the molds are closed.
It is desirable to set the "die height" or preload on the die plates when different molds or other changes are made in the molding machine. The preload is directly related to the amount by which the tie bars are strained when the toggle linkage places the dies in a lock-up condition. Thus, setting the length of the tie bars which are strained between the die plates and thrust plate sets the preload. The effective tie bar length can be set by means of a motor which rotates nuts on the ends of the tie bars and moves the thrust plate relative to the die plates. U.S. Pat. No. 2,498,264 to Goldhard discloses a set of motor-driven nuts of this type and a control mechanism for deenergizing the motor when a particular setting is obtained.
The strain of the tie bars associated with the preload is a small quantity compared to the relative displacement of the die plates in a molding operation; therefore, the mechanism for adjusting that strain must be accurate. In the Goldhard patent referenced above, a limit switch that can be adjusted in position for various preloads is actuated by one of the die plates as the tie bar length for a particular operation is reached. Since the adjustment in length must be set to thousandths of an inch, the limit switch actuated by the die plate, thereby measuring absolute position of the plate, makes accuracy and repeatability difficult to achieve.
It is accordingly a general object of the present invention to provide method and apparatus for setting the preload or die height between two clamped members in molding machines and the like.